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LOAD ALLEVIATION ON WIND TURBINE BLADES USING VARIABLE AIRFOIL GEOMETRY

Thomas Buhl, Mac Gaunaa, Peter Bjørn Andersen and Christian Bak

ADAPWING 1

Outline

• Motivation for the present work

• 2D computations

• Tools

• Main Results

• 3D computations

• Tools

• Main Results

• Wind tunnel testing

• The model

• Preliminary results

• Conclusions

• Future work

Motivation for the work

• State of the art active load reduction employs pitching of whole wing

• Reductions of fatigue loads of up to 28% have been predicted

• But… Very long flexible blades may keep us from pitching fast enough to further reduce fatigue loads

• What if much faster load control could be possible?

• What if local load control on the blade could be possible?

Motivation for the work

• State of the art active load reduction employs pitching of whole wing

• Reductions of fatigue loads of up to 28% have been predicted

• But… Very long flexible blades may keep us from pitching fast enough to further reduce fatigue loads

• What if much faster load control could be possible?

• What if local load control on the blade could be possible?

• Inspiration: Mother nature

• Idea: Use adaptive trailing edge geometry

But why at the trailing edge?

• Potential thin-airfoil theory:

11

21

1

1

1 1

1)(2 dxx

xx

x

yCL

#2: Low loads at TE… Both steady and unsteady.

#1: Maximize bang for bucks

..And why not just a rigid flap?

• Surface discontinuity triggers stall

• Noise issues

• Bad L/D leading to loss in power production

• Flap losing it’s potential load reduction effect

• Go for the continuously

deforming one!

For everything shown here a 10% flap with limits: -5>>5 was used

2D: The tools

• Aerodynamics: Unsteady thin airfoil theory (potential flow) developed

• Modal expansion of the airfoil deflections

• Unsteady terms associated with wake modelled by the computationally efficient indicial method

• Model capable of predicting integral as well as local aerodynamic forces

• Good agreement with attached flow CFD

2D: The tools

• Structural model:

Solid body

+ forces from TE

deformation

• Control: Simple PID control using flapwise deflection as input

2D animation.avi

2D: Main results

• Huge potential fatigue load reduction (~80% reduction of std(N))

• Low time lag essential

• Fast actuation velocity essential

• Trade-off to pitch DOF: Higher fatigue load in torsional direction.

3D model

STRUCTURAL

• Slender cantilever beam theory

• Blade length 33m

• Known structural data

• Mode shapes and eigenfreq. 1f,2f,3f,4f,1e,2e,1Θ,2Θ

AERODYNAMIC

• Turbulent wind series (Veers)

• Induced velocity (Bramwell)

• Dynamic inflow model (TUDk)

• Tip-loss factor (Prandtl)

• Known static lift and drag

• Dynamic flow (Gaunaa)

CONTROL

• Local PID’s on flapwise deflection

• Parameters determined using optimization. min(eq. flapw. root mom.)

3D Results (1)

3D results (2)

3D results (3)

Wind Tunnel Testing

• The Actuator (piezo-electric)

• The Airfoil (Risø B1-18)video from the wind tunnel.wmv

Preliminary result (steady)

Flap side-effect: Very high max lift!

Preliminary result (step flap)

Preliminary result (pitch + flap)

Conclusions

• Big (huge?) load reduction potential

• Time-delays in the system should be avoided at all costs

• Fast actuation velocity important

• Preliminary wind tunnel results look very promising: TE could cancel out lift variations from +-1 pitch motion

Future (and present) work

• Sensoring technique (how to determine the state of the wing dynamically)

• Combined pitch and flap control

• Model aerodynamic dynamic stall effects

• Implement into HAWC2

• What are the implications of this stuff on dynamic stability

• More wind tunnel testing

• More realistic situations (whole span same flap control etc.)